Secondary production and priming reshape the organic matter composition in marine sediments

Organic matter (OM) transformations in marine sediments play a crucial role in the global carbon cycle. However, secondary production and priming have been ignored in marine biogeochemistry. By incubating shelf sediments with various 13C-labeled algal substrates for 400 days, we show that ~65% of the lipids and ~20% of the proteins were mineralized by numerically minor heterotrophic bacteria as revealed by RNA stable isotope probing. Up to 11% of carbon from the algal lipids was transformed into the biomass of secondary producers as indicated by 13C incorporation in amino acids. This biomass turned over throughout the experiment, corresponding to dynamic microbial shifts. Algal lipid addition accelerated indigenous OM degradation by 2.5 to 6 times. This priming was driven by diverse heterotrophic bacteria and sulfur- and iron-cycling bacteria and, in turn, resulted in extra secondary production, which exceeded that stimulated by added substrates. These interactions between degradation, secondary production, and priming govern the eventual fate of OM in marine sediments.


Supplementary Text
Quantification of the carbon content of 13 C-algal lipids and 13 C-algal proteins To quantify the carbon content of fully 13 C-labeled substrates, 2 parts of the substrate were mixed with 98 parts of glucose followed by measurement via EA-IRMS.The carbon content of the substrates was calculated based on equation ( S1 DI 12 Ct can be calculated from the incubation group and it is the sum of 12 C-DIC priming and 12 C-DIC control. 12C-DIC control is the DIC production from the control group where no substrate was provided. Quantification of 13 C-amino acids (AA) production at time t f is defined as the 13 C content of all amino acids: Quantification of biomass based on 13 C-amino acids (AA) production at time t AA 13 Ct equals the newly produced protein and accounts for around 60% of newly produced biomass, 13 (S1)   =   +   (S2) with M defined as the weight (mg) of different organic compounds.Quantification of DIC production at time t   =   −  0 (S3) Quantification of DI 13 C production at time t    13 =   × (  −  0 ) (S4)f is defined as the 13 C content of DIC: Fig. S1.δ 13 C of CO2 indicates the turnover of added organic substrates during 400-day-long incubations.Development of δ 13 C-CO2 in the headspace during incubations with algal substrates ( 13 C-algal lipids or 13 C-algal proteins) and the control group without substrate in two depths of Helgoland mud area sediment.

Fig
Fig. S2.δ 13 C of DIC indicates the turnover of added organic substrates during 400-day-long incubations.Development of δ 13 C-DIC in the liquid phase during incubations with algal substrates ( 13 C-algal lipids or 13 C-algal proteins) and the control group without substrate in two depths of Helgoland mud area sediment.

Fig. S3 .
Fig. S3.DIC indicates the turnover of organic matter in the system during 400-day-long incubations.Development of total DIC concentration during incubations with algal substrates ( 13 C-algal lipids or 13 C-algal proteins) and the control group without substrate in two depths of Helgoland mud area sediment.

Fig. S4 .
Fig. S4.Distributions of 13 C-labeled amino acids indicate mixed signals of substrate turnover and secondary production.The compositional change of 13 C-labeled amino acids (normalized to "μmol C" to account for different carbon numbers) during the 13 C-algal protein incubations.

Fig. S5 .
Fig. S5.RNA distribution in different density fractions during the 13 C-algal lipid and protein incubations.Std indicates the RNA standard containing an equal amount of 12 C-and 13 C-labeled nucleic acids from E. coli.The mixed area between vertical dashed lines indicates fractions that are partially 13 C-labeled.

Fig. S6 .
Fig. S6.Bacterial community composition during lipid and protein degradation.RNA density distribution targeting algal lipid and protein bacterial degraders during the 13 C-algal lipid and protein incubations.

Fig. S7 .
Fig. S7.The δ 13 C value archaeal lipid derivatives indicate the activity of archaea.The change in carbon isotopic compositions of different biphytanes (BP) derived from archaeal lipids during the 13 C-algal lipid and protein incubations in two depths of Helgoland mud area sediment.The number behind BP indicates the number of cyclopentane rings in the molecules.

Fig. S8 .
Fig. S8.Archaeal community composition during lipid and protein degradation.RNA density distribution targeting algal lipid and protein archaeal degraders during the 13 C-algal lipid and protein incubations.The active archaea are dominated by Asgardarchaeota, Woesearchaeales, and SG8-5.The sequencing data was not retrieved from all experiments due to low RNA recovery.

Fig. S9 .
Fig. S9.Extra production of DI 12 C indicates the priming intensity.The differences in DI 12 C production between incubations with 13 C-algal lipid substrate and the control group without substrate is a clear signal of priming.